1. Field of the Invention
The present invention relates to a node device, and in particular to a node device which composes a communication system with a terminal, or with a terminal and a location registering server.
Recently, a next generation mobile communication system has been studied in research institutions, standardization groups, and the like all over the world. The next generation mobile communication system aims at offering multi-media services including a high-speed data communication to the extent of 2 Mbps.
Particularly, it is expected that packet communication services including the Internet will become the core services of the next generation of mobile communications in lieu of present services mainly using voice.
Furthermore, the study of the communication system has started where a mobile and a fixed station are integrated. In such an integrated communication system, it is important to reliably transmit a packet from a source terminal to a destination terminal.
2. Description of the Related Art
Current concept diagrams and problems of a mobile communication packet system realized by a cellular communication network in Japan and Europe are mentioned in a mobile packet communication system of the U.S. patent application Ser. No. 09/482,712 (filed on Jan. 13, 2000). This entire application is hereby incorporated by reference.
FIG. 32 shows an arrangement of the mobile packet communication system in the above-mentioned application. This communication system is composed of location registering servers 30_1 and 30_2, and node devices 20_1–20_4, each node device having a respective area 21_1–21_4 of communication.
Addresses [ADR1]–[ADR6] are assigned to the location registering servers 30_1, 30_2, and the node devices 20_1–20_4.
Furthermore, the communication system includes mobile terminals (hereinafter simply referred to as terminals) 10_3, 10_4, 10_1, and 10_2, which are respectively located in the areas 21_1–21_3, and addresses [1.1.1], [1.1.2], [2.1.1], and [2.2.1] are respectively assigned to each of the terminals 10_1–10_4.
It is to be noted that in FIG. 32 a subscriber node and a gate node in the above-mentioned application are not distinguished therebetween and simply referred to as a node device. In addition, reference numerals or characters of a terminal, a node device, and the like are used in modified forms.
The location registering servers 30_1 and 30_2 are established corresponding to an upper figure of a destination address (e.g. IPv4 and IPv6) of the packet, and are respectively provided with location registering tables 31_1 and 31_2 which register the address of the terminal corresponding to the address of the node device having the area where the terminal is located.
For instance, the addresses [1.1.1] and [1.1.2] of the terminals 10_1 and 10_2, the address of which is represented by [1.X.X (“X”: don't care)], are made corresponding to the address [ADR5] of the node device 20_3 having the area where the terminals 10_1 and 10_2 are located to be registered in the location registering table 31_1.
Similarly, the addresses [2.1.1] and [2.2.1] of terminals 10_3 and 10_4, the address of which is represented by [2.X.X], and the addresses [ADR3] and [ADR4] of the node devices 20_1 and 20_2, are registered in the location registering table 31_2.
Also, the servers 30_1 and 30_2 are provided with a function of transferring the packet based on the information of the location registering tables.
The node devices 20_1–20_4 have location information tables 22_1–22_4 (only 22_2 is shown) for storing the addresses of the terminal and the destination node device, and a server retrieval table 40 for retrieving the address of the corresponding location registering server from the upper figure of the destination address of the packet.
FIG. 33 shows an operation example of the packet transfer in the mobile packet communication system shown in FIG. 32. Hereinafter, the operation when a user packet 71 is transmitted from the terminal 10_4 to the terminal 10_1 will be described in the order of processing steps.    Step S1: The terminal 10_4 transmits the user packet 71 including the address [1.1.1] indicating “addressed to the terminal 10_1” to the node device 20_2.    Step S2: The node device 20_2 retrieves the address [ADR1] of the server 30_1 corresponding to the address [1.1.1] of the terminal 10_1 from the server retrieval table 40.    Step S3: The node device 20_2 adds the address [ADR1] of the server 30_1 to the received user packet 71 to be transferred to the server 30_1 as an inter-node packet 81.    Step S4: The server 30_1 retrieves the address [ADR5] of the node device 20_3 having the area where the terminal 10_1 is located from the location registering table 31_1.    Step S5: The server 30_1 transfers an inter-node packet 82, in which the destination address of the received packet 81 is rewritten to [ADR5], to the node device 20_3.    Step S6: The server 30_1 notifies the node device 20_2 of the address [ADR5] of the node device 20_3 having the area where the terminal 10_1 is located based on a source address [ADR4] of the packet 81.    Step S7: The node device 20_3 transmits the packet 71, that is the packet 82 from which the address [ADR5] of its own device is removed, to the terminal 10_1, so that the terminal 10_1 receives the same.    Step S8: The node device 20_2 updates the address of the node device having the area where the terminal 10_1 is located in the location information table 22_2 to [ADR5] or adds [ADR5] to the node device address.    Step S9: The next user packet 71 addressed to the terminal 10_1 arrives at the node device 20_2.    Step S10: The node device 20_2 retrieves the location information table 22_2 to obtain the address [ADR5] of the node device 20_3 having the area where the terminal 10_1 is located.    Step S11: The node device 20_2 directly transfers to the node device 20_3 an inter-node packet 83, that is the received packet 71, addressed to the terminal 10_1 and added with the address [ADR5] of the node device 20_3.    Step S12: The node device 20_3 transmits the user packet 71, that is the packet 83 of the node device 20_3 from which the address [ADR5] is removed, to the terminal 10_1.
Thus, the packets 71 which the terminal 10_4 subsequently transmits are received by the terminal 10_1.
In such a communication system composed of such a node device, a packet loss occurs when the terminal 10_1 moves to an area of another node device.
This will be described by referring to FIG. 34 which shows an operation after a packet transfer operation of FIG. 33.    Step S1: The terminal 10_1 moves to the area 21_4 of the node device 20_4.    Step S2: The terminal 10_1 makes a location registration to the communication system.    Step S3: The node device 20 4 retrieves the address [ADR1] of the location registering server 30_1 corresponding to the address [1.1.1] of the terminal 10_1 from the server retrieval table 40.    Step S4: The node device 20_4 transmits location registering information to the location registering server 30_1.    Step S5: The location registering server 30_1 updates the address of the node device having the area where the terminal 10_1 is located in the location registering table 31_1 to [ADR6].    Step S6: The user packet 71 addressed to the terminal 10_1 arrives at the node device 20_2.    Step S7: The node device 20_2 retrieves the address [ADR5] of the node device 20_3 having the area where the terminal 10_1 is located from the location information table 22_2.    Step S8: The node device 20_2 transfers to the node device 20_3 the inter-node packet 81, that is the received packet 71, addressed to the terminal 10_1 and added with the address [ADR5].    Step S9: Since the node device 20_3 does not have the area where the destination terminal 10_1 is located, the user packet 71 cannot be transmitted to the terminal 10_1.
Hereafter, every time the terminal 10_4 transmits the user packet 71 to the terminal 10_1, steps S6–S9 are repeated, so that the user packet 71 is discarded.
As mentioned above, when the destination terminal moves to a different destination node device in such a mobile packet communication system, the user packet received at a former node device cannot be delivered to the destination terminal, so that a communication quality deteriorates due to the occurrence of packet loss.
Also, since the location information of the destination terminal cached in the location information table of a source node device is not updated, there has been a problem that the packet transfer to the former node device is inevitably continued although the destination terminal has already moved.